1. Field of the Invention
The present invention relates generally to hydraulic presses, such as press brakes for working or processing sheet-like workpieces, such as sheet metals, and pertains more particularly to hydraulic presses which are equipped with a pair of long upper and lower tools either of which is hydraulically operated to work or process sheet-like workpieces in cooperation with each other.
2. Description of the Prior Art
Some hydraulic presses, such as those which are referred to as hydraulic press brakes, are equipped with a pair of long upper and lower tools to work or process sheet-like workpieces, such as sheet metals, mainly to bend them into shapes, such as channels and angles. In such hydraulic presses, either of the long upper and lower tools is horizontally fixed to a beam member and the other of them is horizontally mounted on a beam-like ram member which has a horizontally long holding portion and is hydraulically operated. In order to drive the movable tool which is long in shape, the beam-like ram member is so arranged as to be driven vertically at its ends by a pair of hydraulic motors of cylinder types into and out of which hydraulic motors of cylinder types into and out of which hydraulic fluid is delivered and exhausted. In this arrangement, the beam-like ram member is vertically driven to drive the movable tool vertically toward and away from the fixed tool to work or process a sheet-like workpiece placed between the movable and fixed tools when the hydraulic motors are supplied with the hydraulic fluid. Also, the hydraulic motors are controlled to enable the ram member to stop the movable tool at a distance from the fixed tool according to the thickness, width and tensile strength of the workpiece to be worked and the shape or angle into which the workpiece is to be worked or bent.
In the conventional hydraulic presses of the kind described above, the beam-like ram member will often fail to move equally both ends of the long movable tool in such a manner as to keep the ends always at a level with each other. In other words, the beam-like ram member will often fail to evenly move in equilibrium in order to evenly drive the movable tool with the ends thereof kept always at a level with each other. Therefore, the movable tool will become oblique or offset relative to the fixed tool. The beam-like ram member will almost inevitably fail to evenly move in equilibrium from various causes, such as a difference between frictions or sliding resistances of guide means at the ends of the ram member and a difference between fluid resistances in the hydraulic motors and their hydraulic circuits. Furthermore, even if the beam-like ram member can evenly move in equilibrium, the movable and fixed tools will become oblique to each other, since the beam-like ram member and the beam member holding the movable and fixed tools, respectively, will deflect from each other from various causes. For instance, the beam-like ram member and the beam member will deflect from each other when a narrow workpiece is being worked or bent at other than central portions of the movable and fixed tools and when the workpiece is uneven in tensile strength or in thickness, even if worked or bent at the central portions of the movable and fixed tools. As a matter of course, the workpiece cannot be accurately worked or processed if either of the movable and fixed tools becomes oblique to the other, much less if both of them become oblique to each other. For instance, if either of the movable and fixed tools is oblique to the other, the sheet-like workpiece cannot be equally bent as a whole and will be bent into different angles at portions and especially at the opposite ends.
Another disadvantage with the conventional hydraulic presses has been that it is very difficult and time-consuming to accurately adjust the moving limit of the movable tool to the fixed tool, i.e. the shortest distance between the movable and fixed tools according to the thickness, width and tensile strength of workpieces to be processed. Of course, the movable and fixed tools cannot work accurately and will often break or prematurely wear unless the moving limit of the movable tool to the fixed tool is accurately adjusted.